Method for the purification of zinc oxide controlling particle size

ABSTRACT

A method for the recovery of high purity zinc oxide products, and optionally iron-carbon feedstocks, from industrial waste streams containing zinc oxide and/or iron. The waste streams preliminary can be treated by adding carbon and an ammonium chloride solution, separating any undissolved components from the solution, displacing undesired metal ions from the solution using zinc metal, treating the solution to remove therefrom zinc compounds, and further treating the zinc compounds and the undissolved components, as necessary, resulting in the zinc products and the optional iron-carbon feedbacks. Once the zinc oxide has been recovered, the purification process is used to further purify the zinc oxide to obtain zinc oxide which is at least 99.8% pure and which has predeterminable purity and particle characteristics. Various zinc compounds may then be quickly, easily, and economically produced from this recovered zinc oxide.

STATEMENT OF RELATED APPLICATIONS

This application is a divisional of application Ser. No. 08/594349 filedon Jan. 29, 1996, now U.S. Pat. No. 6,696,029, which is acontinuation-in-part of application Ser. No. 08/439352 filed on May 11,1995, which issued as U.S. Pat. No. 5,759,503 on Jun. 2, 1998, which isa continuation-in-part of application Ser. No. 08/238250 filed on May 4,1994, which issued as U.S. Pat. No. 5,464,596 on Nov. 7, 1995, which isa continuation-in-part of application Ser. No. 07/953645 filed on Sep.29, 1992, abandoned, which is a continuation-in-part of application Ser.No. 07/820987 filed on Jan. 15, 1992, which issued as U.S. Pat. No.5,208,004 on May 4, 1993.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Many of the uses of zinc oxide require that the zinc oxide have certainparticular size, shape and purity characteristics. Therefore, manygrades of zinc oxide having different purity and particlecharacteristics have been developed to meet the diverse industryrequirements. Today, most zinc oxide is made by the so called FrenchProcess which involves controlled burning of zinc metal vapor in air toobtain zinc oxide having exceptional chemical purity. The presentinvention provides a zinc oxide purification process which involvesprecipitating zinc oxide in such a manner that the desired purity andparticle characteristics can be obtained. One method to control particlesize is through the control of the conditions of the washing step.Additionally, although the zinc oxide purification process preferablyutilizes a sodium hydroxide solution as the intermediate, thepurification process of the present invention also provides forpreparation of zinc oxide having particular purity and particlecharacteristics by utilizing intermediates such as ammonium chlorideliquor, ammonium sulfate, ammonium phosphate, potassium hydroxide,ammonia/ammonium oxalate and ammonia/ammonium carbonate solutions. Oncethe zinc oxide has been dissolved in the solution, controlled dilutionresults in the precipitation of zinc oxide having predetermined purityand particle characteristics.

The present invention relates generally to a process for the recovery ofzinc products including essentially pure zinc oxide and, optionally, aniron-carbon residual from industrial waste streams comprising zinccompounds and iron compounds. The present invention relates morespecifically to a process subjecting a waste materials stream comprisingzinc compounds and iron compounds, such as electric arc furnace (EAF)dust, to a combination of leaching and reducing steps, for the recoveryof essentially pure zinc oxide in a recycling operation which recyclesprocess solutions for reuse, and produces a cake product fromundissolved iron and carbon compounds which can be used as a feedstockfor steel mills. Once the essentially pure zinc oxide has beenrecovered, the zinc oxide is further purified by a process which ispreferably based on the solubility of zinc oxide in a concentratedsodium hydroxide solution. This final purification process can becontrolled in such a manner that the particle size and surface area ofthe zinc oxide produced can be controlled. Additionally, zinc compoundscan be quickly, easily, and economically, synthesized from the aqueouszinc oxide slurry resulting from this process.

2. Prior Art

Zinc oxide typically is a coarse white or grayish powder which has avariety of uses including as an accelerator activator, as a pigment, asa dietary supplement and in the semiconductor field. Zinc oxide is foundin commercial by-products including waste material streams such as flyash and flue dust. Methods for recovering zinc oxides are known in theart, including recovering zinc oxide from industrial waste materials.Such previous methods have included leaching with mineral acid, causticsoda, ammonium hydroxide, and ammonium carbonate solutions. However,these methods have low yields of zinc oxide and typically do not recoverpure zinc oxide, the recovered zinc oxide being contaminated with othermetal salts. Therefore, in order to obtain pure zinc oxide, subsequentroasting and evaporation processes were necessary.

U.S. Pat. No. 3,849,121 to Burrows, now expired but which was assignedto a principal of the assignee of the present invention, discloses amethod for the selective recovery of zinc oxide from industrial waste.The Burrows method comprises leaching a waste material with an ammoniumchloride solution at elevated temperatures, separating iron fromsolution, treating the solution with zinc metal and cooling the solutionto precipitate zinc oxide. The Burrows patent discloses a method to takeEAF dust which is mainly a mixture of iron and zinc oxides and, in aseries of steps, to separate out the iron oxides and waste metals.However, the material obtained in the last step is a mixture of a smallamount of zinc oxide, hydrated zinc phases which can include hydrates ofzinc oxide and zinc hydroxide, as well as other phases and a largeamount of diamino zinc dichloride Zn(NH₃)₂Cl₂ or other similar compoundscontaining zinc and chlorine ions. Currently, the Burrows method is noteconomically viable because of Environmental Protection Agencyguidelines established subsequent to the issuance of the Burrows patent.Additionally, the Burrows method is not a continuous method and,therefore, is not economical as a continuous process.

The first step in the Burrows patent is the treating of the EAF dustwith an ammonium chloride solution. The action of the treatment is theleaching of zinc oxide, lead oxide and cadmium oxide in the solutionwithout any leaching of the iron oxides present. Twenty to fifty percentof the zinc present in the Burrows dust is in the form of an iron-zinccomplex (known as a spinel) which cannot be leached by the ammoniumchloride solution. The Burrows process therefore cannot leach andrecover a significant portion of zinc present in the EAF dust.

The second step in the Burrows process is cementation in which thesolution obtained from the initial leach is filtered to remove anyremaining solids and then zinc dust is added. The zinc dust causes anelectrochemical reaction which causes the lead and cadmium to deposit onthe zinc particles. Burrows does not teach the need to remove the leadand cadmium in this step efficiently without using a large amount ofzinc. If the process requires too much zinc in this step, it will not beeconomically viable. The zinc powder when added tends to clump togetherreducing the available surface area and requiring the addition of morezinc.

The third step in the Burrows patent then takes the filtrate from thecementation process and cools the filtrate and obtains what are called“zinc oxide” crystals. Burrows indicates that these crystals range insize up to ⅜ of an inch. Burrows does not produce zinc oxide of anydegree of purity; x-ray diffraction figures clearly show that uponcrystallization there is a mixture of many phases. Washing the crystalsis not sufficient to purify the material to zinc oxide since zinchydroxide and hydrates are also present, so that a drying step isnecessary. In addition, the control of the size of the zinc oxide alongwith the purity is crucial. Commercial zinc oxide normally has arequirement that 99% of the particles fit through 325 mesh (44 microns).Burrows indicates no method of cooling or controlling either purity orsize, and the particles produced do not meet commercial requirements.Further, a significant portion of the ammonium chloride is lost in thecrystal washing step when the diamino zinc dichloride decomposes.

Waste metal process dust typically has varying amounts of lead, cadmiumand other metals contained in the dust. For various reasons, it isdesirable to remove such metals from the waste metal dust, for exampleto recycle the lead and cadmium and/or to prevent introduction of thelead and cadmium into the atmosphere. The Burrows patent includes amethod for removing dissolved lead and cadmium from the ammoniumchloride solutions which have been used to treat the waste metal dust bythe addition of powdered zinc dust to the ammonium chloride solutions.The resulting electrochemical reaction forms elemental lead deposits onthe surface of the powdered zinc dust. For this reaction to proceed, alarge surface area of zinc initially must be present because as the leadcovers the zinc dust particle, the particle becomes no longer availablefor the electrochemical reaction. For this reason, very fine powder isused which, unfortunately, immediately aggregates to form large clumpswhich sink to the bottom of the vessel. Rapid agitation does not preventthis from happening. Because of the aggregation of zinc, a large amountof zinc must be added to remove all of the lead, a poor practice foreconomic reasons. Further, if it is desired to separate the lead andsome cadmium from the zinc so that all of these metals can be sold orreused, the higher the zinc concentration in the metals, the larger themass to be processed per unit mass of zinc.

U.S. Pat. No. 4,071,357 to Peters discloses a method for recoveringmetal values which includes a steam distillation step and a calciningstep to precipitate zinc carbonate and to convert the zinc carbonate tozinc oxide, respectively. Peters further discloses the use of a solutioncontaining approximately equal amounts of ammonia and carbon to leachthe flue dust at room temperature, resulting in the extraction of onlyabout half of the zinc in the dust, almost 7% of the iron, less than 5%of the lead, and less than half of the cadmium.

Steam distillation is directly contrary to temperature lowering; steamdistillation precipitates zinc carbonate, other carbonates and ironimpurities, whereas temperature lowering advantageously precipitates anumber of crystalline zinc compounds. Steam distillation alsodisadvantageously results in an increase in temperature which drives offammonia and carbon dioxide, resulting in the precipitation of ironimpurities and then zinc carbonate and other dissolved metals. Thepurity of the zinc carbonate obtained depends on the rate of steamdistillation and the efficiency of solids separation as a function oftime. Calcining at temperatures between 200° C. and 1100° C. convertsthe zinc carbonate to zinc oxide, whereas washing and drying attemperatures between 100° C. and 200° C. converts the zinc compounds tozinc oxide. In addition to the advantages of temperature lowering, thepresent process also employs a 23% NH₄Cl solution at temperaturesranging from 90-110° C., and has several distinct advantages over thePeters process:

1. The solubility of zinc and zinc oxide is relatively high in NH₄C1solution which is important to the efficiency of the present process interms of the rate of the leaching, the mass of dust that can beprocessed, and the ability to recycle the solution. The rate of theleaching (which is a dissolution process) is a function of thedifference between the zinc concentration in solution and the saturationconcentration; the higher the saturation concentration the more rapidthe leaching. The present process leaches for only 1 hour, while thePeters process leaches for at least several hours. In addition, theammonium chloride solution has the added property that the solubility ofzinc and zinc oxide in the solution declines rapidly with temperature,which is the basis for the crystallization-based separation which isused later in the present process.

2. Lead and lead oxide, as well as cadmium and cadmium oxide, aresoluble in the ammonium chloride solution while iron oxide is virtuallyinsoluble. During the leaching process of the present invention, 95-100%of the zinc present as zinc oxide is extracted, compared to about 55% inPeters; 50-70% of the lead present is removed, compared to less than 5%in Peters; and 50-70% of the cadmium is removed, compared to less thanhalf in Peters. In effect, Peters does not remove a significant amountof the impurities so as to leave an acceptably clean effluent. Petersindicates that his residue, which is high in lead and is a hazardouswaste, is discarded. By leaching out a significant portion of the leadand cadmium, the present process produces a material which can be usedby the steel producer as they use scrap metal.

3. Peters adds powdered zinc to the solution, which has a tendency toclump reducing the surface area available for the dissolution of thezinc and the plating of the lead and cadmium. The present processteaches a method to minimize this effect through the use of an organicdispersant.

In the present process the filtrate from the cementation step is alreadyhot (90-110° C.) and contains a large amount of dissolved zinc withsmall amounts of trace impurities. Upon controlled cooling of thesolution, crystals of zinc salts begin to appear. Control of the coolingrate and temperature versus time profile is important in controlling thesize distribution of the crystals and in reducing or eliminating many ofthe impurities which might occur. This is especially true of includedsolution; control of the crystallization can reduce this to virtuallyzero. In addition, since crystallization is based on differentialsolubility, and none of the impurities is present in a concentrationwhich can crystallize, the zinc salts are virtually free of any metalimpurities.

The final purification step in Peters is a calcining of the zinccarbonate at 600° C. to zinc oxide. In the present process, the mixtureof zinc oxide hydrates and diamino zinc dichloride are suspended in hot(90-100° C.) water. The zinc oxide is not soluble; however, the diaminozinc dichloride is very soluble and completely dissolves. The remainingsolid which is zinc oxide hydrates is then filtered and dried at100-350° C. to remove the water of hydration. The result is a very purezinc oxide powder of controlled particle size.

Another process offered by Engitec Impianti SpA, of Milan, Italypurports to recover zinc metal and lead cement directly from EAF fluedust using an electrowinning technology. Electrowinning is the techniqueof extracting a metal from its soluble salt by an electrolytic cell.Typically, it is used in the recovery of zinc by subjecting the zincsalt in solution to electrolysis and electrodepositing the elementalmetal on a zinc cathode in the electrolytic cell. In the Engitecprocess, the EAF flue dust is leached with a spent electrolyte, such asammonium chloride, which dissolves the zinc, lead, copper and cadmium inthe EAF dust into solution while leaving the iron in solid form. Thesolution containing the dissolved zinc is placed in an electrolytic cellwhich draws the zinc from the solution onto a cathode plate, while theother heavy metals are filtered out in solid form into cement cakes.Engitec claims to obtain a zinc yield that is 99.5% pure and a lead cakeconsisting of a minimum of 70% lead. In effect, the Engitec processtakes the product solution from the Burrows process and subjects it toelectrowinning. A neutral solution of ammonium and sodium chlorides isheated to between 70° C. and 80° C. The EAF dust is mixed into thechlorides solution in which the zinc and heavy metals are dissolved. Theiron, calcium, magnesium and aluminum oxides are insoluble in thechlorides solution. After leaching and residue filtration, the solutionis purified by cementation using zinc granules or powder. After removalof the cement, consisting of lead, copper, silver and cadmium, thepurified solution is fed to the electrolysis cell.

Apparently, the electrolysis of the zinc amino complex in the purifiedsolution occurs in a conventional open cell using a titanium permanentblank cathode and a proprietary graphite anode. In the electrolysiscell, the zinc plates on the titanium cathode. However, the depositiontime for the zinc is 24 to 48 hours, depending on the current density.In addition to the electrowinning of zinc, the electrolysis cellconsumes ammonia and evolves nitrogen. Because of this, in order tomaintain the pH of the electrolyte in the desired range of 6 to 6.5,additional ammonium must be added to the cell in the range of 180 kg perton of product zinc.

Although the Engitec process appears to be theoretically possible, theuse of an electrolysis cell adds additional costs to the process due tothe energy consumption of an electrolysis cell, the consumption ofammonia in the electrolysis cell, additional costs in handling nitrogenevolved in the electrolysis cell, and the cost of maintaining thecomponents of the electrolysis cell itself. For example, the titaniumcathode can be costly, while the apparently proprietary graphite anodealso may be costly. The Engitec process also results in the formation ofmetallic zinc. Although metallic zinc has a certain value, zinc oxidehas more value. The residue removed from the Engitec process is composedprimarily of iron oxide and zinc ferrite. Iron oxide can be used in thesteel making process. The presence of zinc ferrite likely is not asignificant detriment to the use of the residue from the Engitecprocess, but it does inject an additional impurity into any futureprocess. It would be more advantageous to obtain a residue comprisingprimarily iron oxide with no zinc ferrite or other impurities, or onlyan insignificant amount of such other impurities.

The electrowinning of metals from chloride solutions is known in theart. U.S. Pat. No. 4,155,821 to Grontoft discloses and claims a methodfor recovering chlorine using electrolytic separation. Chlorine andmetal are produced from a chlorine containing electrolyte by anelectrolytic process having an anode surrounded by a membrane connectedto a hood. The process is maintained at a partial vacuum so that anychlorine gas generated by the anode together with some of theelectrolyte is drawn away from the anode. The vacuum also is devised tocontrol redifussion of chlorine containing electrolyte back through themembrane into the surrounding electrolyte. The process is for use withnickel recovery where the nickel chloride containing electrolyte isintroduced at such a rate that the pH is maintained below a certainlevel. The process also may be used for cobalt recovery.

The electrodeposition of zinc from chloride solutions also is known inthe art. U.S. Pat. No. 4,292,147 to Fray discloses and claims a methodfor the electrodeposition of cadmium or zinc from chloride solutionsderived from chlorine leaching of materials. An aqueous solution having15 to 30% by weight of zinc or cadmium chloride is electrolyzed at a pHof 2 to 3.5 at a temperature of below 35° C. with gas agitation at acurrent density above 100 A/m² to form coherent zinc or cadmium at thecathode. A typical zinc containing material such as flue dust is leachedwith a saturated chlorine solution, preferably in the presence ofchlorine hydrate. The zinc chloride solution preferably contains 20 to30% by weight zinc or cadmium chloride and up to 20% by weight alkalinemetal or ammonium chloride. The electrolysis preferably is carried outat 0° C. to 9° C. and above 2500 A/m² with intermittent currentreversal. Chlorine hydrate liberated at the anode may be recycled toaffect leaching.

In addition to the uses of zinc oxide as a pigment, there are severalimportant zinc compounds which are synthesized from zinc oxide. Methodsto make various zinc compounds from zinc oxide are well known in theart. The methods usually involve the reaction of zinc oxide with theappropriate acid to form the zinc compound. Generally, commercial zincoxide is first made by the combustion of zinc vapor in air, resulting indry powdered zinc oxide. This dry powder must first be suspended ordissolved in an aqueous solution prior to treatment with the appropriateacid to produce the desired compound. In many cases the zinc oxide isdissolved in an aqueous solution of an acid. The zinc compounds may bepresent in solution or alternatively may be precipitated as a solid.

Thus, there exists a need for a method which will recover high purityzinc oxide products from industrial waste streams. The method disclosedbelow relates to the preparation of essentially pure zinc oxide. Inaddition, since zinc oxide is the desired product and diamino zincdichloride is undesired, the method disclosed herein demonstrates how toincrease the formation of desired zinc products and decrease theformation of diamino zinc dichloride. Furthermore, once the essentiallypure zinc oxide has been obtained, the zinc oxide is further purified bya process which is based on the solubility of zinc oxide in aconcentrated sodium hydroxide solution. This purification process can becontrolled to produce zinc oxide having a desired size and surface area.

There also exists a need for a method which will allow the recovery ofiron oxide from industrial waste streams which can be used with littleor no additional treatment as the feedstock for other processes.Producing an iron oxide with a minimum amount of impurities, such aszinc ferrite, is advantageous because the iron oxide can be used as thefeedstock for steel production processes. A method which results in therecovery of iron oxide would have additional value in that the ironoxide could be sold for use in other processes.

There also exists a need for a quick, easy, and economical method forthe synthesis of various zinc compounds from recovered zinc oxide.

Additionally, with specific regard to this disclosure, there exists aneed for methods of purifying zinc oxide which allows a highly purifiedzinc oxide to be obtained which has predeterminable and controllablepurity, and particle size and shape characteristics.

BRIEF SUMMARY OF THE INVENTION

The present invention satisfies these needs in a method which recoversessentially pure zinc oxide ultimately from waste material containingzinc or zinc oxide. Along with the essential pure zinc oxide, zinc metalcan be recovered, along with values of other metallic elements containedin the waste material such as lead, silver, and cadmium. The solutionsused in the process are recycled such that the process does not have anyliquid wastes. The solids recovered from the process, namely, the zincoxide, zinc, metal values, and other residues all can be used in otherprocesses. One such residue, an iron oxide cake, is of such a qualitythat it can be used directly as the feedstock for the typical steelproduction process. The zinc oxide recovered in this preliminary processis further purified by the preferred process which is based on thesolubility of zinc oxide in a concentrated sodium hydroxide solution.This purification process can be controlled to produce zinc oxide havinga desired size and surface area.

One method for purifying zinc oxide to obtain zinc oxide crystals havingpredetermined purity and particle characteristics comprises the steps ofdissolving a zinc oxide containing product in an intermediate, filteringout any undissolved materials, precipitating zinc oxide crystals out ofthe intermediate in a controlled manner such that the zinc oxidecrystals have predetermined purity and particle characteristics,filtering out the zinc oxide crystals, washing the zinc oxide crystals,and then drying the zinc oxide crystals.

The intermediate is preferably selected from the group consisting ofsodium hydroxide, ammonium sulfate, ammonium chloride liquor, ammoniumphosphate, potassium hydroxide, ammonia/ammonium oxalate, andammonia/ammonium carbonate solutions. Most preferably, the intermediateis a concentrated 50%-70% sodium hydroxide solution. The precipitationstep is accomplished by diluting the solution at a predetermined rate bya factor ranging from 3 to 30 at a temperature ranging from about 25° C.to 100° C. at atmospheric pressure, and even over 100° C. at pressuresgreater than atmospheric pressure, to precipitate the zinc oxidecrystals.

The preferred method for purifying zinc oxide to obtain zinc oxidecrystals having predetermined purity and particle characteristicscomprises controlling the method by which the intermediate, preferablysodium hydroxide, is added to the zinc oxide can be used to control theparticle size of the resulting purified zinc oxide. Preferably, thesodium hydroxide is dispersed into droplets averaging in size from 100to 300 microns, with 150-250 microns being preferred, and the bestresults being obtained at 180 microns. Generally, the smaller thedroplet size the larger the surface area of the resulting zinc oxideparticles.

Once the substantially pure zinc oxide is recovered from the preliminaryprocess, the purification process takes place resulting in zinc oxidewhich is at least 99.8% pure. In the preferred embodiment, thispurification process is based on the solubility of zinc oxide in aconcentrated sodium hydroxide solution. In the preferred process, zincoxide is dissolved in a concentrated 50%-70% sodium hydroxide solution.Most of the metal impurities contained in the zinc oxide will notdissolve, including manganese, iron and cadmium. Any lead, calcium orchloride contained in the zinc oxide will dissolve. The solution is thenfiltered to remove the undissolved solids, which are then recycled backto the metals recovery section of the plant and thereby returned to therecycling process of the present invention.

The solution is then diluted by a factor ranging from 3 to 30, andpreferably 3 to 8. The dilution preferably is performed hot attemperatures at or above 70° C., preferably at temperatures rangingbetween 90 to 100° C., so that the dilution step favors the formation ofzinc oxide as compared to zinc hydroxide. The zinc oxide crystals whichform are then filtered out, sent to a wash tank where they are washedwith water, and sent to a dryer where they are dried, preferably at atemperature of 160° C.

The diluted sodium hydroxide solution then is sent to an evaporatedcondenser where the solution is concentrated back to 50%-70% sodiumhydroxide so that it can be reused. When a steady state has beenachieved, this step will result in the formation of sodium chloridecrystals which will be filtered out of the solution and recovered. Thisis because sodium chloride formed by the chloride present in the zincoxide is less soluble in a concentrated sodium hydroxide solution thanin dilute sodium hydroxide. After the sodium chloride is filtered out,the concentrated solution can be reused in the purification process ofthe present invention.

Periodically, lead will be removed from the sodium hydroxide solution bycementation. This involves the addition of zinc dust which will displacethe lead in solution. The lead will be filtered out and sent to the leadrecovery portion of the plant.

By controlling the manner in which the zinc oxide precipitates out ofthe intermediate during the zinc oxide crystallization step, it ispossible to control the particle size hence the surface area of the zincoxide produced as well as the purity. Furthermore, the purificationprocess of the present invention can be used to purify zinc oxideobtained from other sources.

Therefore, it is an object of the present invention to provide a methodfor recovering zinc oxide from waste materials, such as fly ash or fluedust, which contain other metals, such as iron oxide, lead oxide,cadmium and other materials.

Yet another object of the present invention is to provide a method forrecovering zinc oxide in which all leaching and washing solutions arerecycled for further use, and no leaching or washing solutions aredisposed of into the sewers or the environment.

Still another object of the present invention is to provide a method forrecovering zinc oxide which also results in the precipitation inelemental form of any lead and cadmium metals contained in the startingmaterials.

It is another object of the present invention to provide a method forrecovering zinc oxide in which all of the zinc can be recycled so thatall of the zinc eventually will be converted to zinc oxide.

Still another object of the present invention is to provide a method forrecovering iron oxide from waste materials, such as fly ash or fluedust, which contain other metals, such as zinc, lead oxide, and cadmium.

A further object of the present invention is to provide a method forrecovering iron oxide which can be used as is as a feedstock for steelproduction processes.

Another object of the present invention is to provide a method forrecovering zinc metal, zinc oxide and/or iron oxide which is economical,quick and efficient.

A further object of the present invention is to provide methods ofpurifying zinc oxide which allows a highly purified zinc oxide to beobtained which has predeterminable and controllable purity, and particlesize and shape characteristics.

A final object of the present invention is to provide a quick, easy, andeconomical method for the synthesis of various zinc compounds fromrecovered zinc oxide.

These objects and other objects, features and advantages of the presentinvention will become apparent to one skilled in the art when thefollowing Detailed Description of a Preferred Embodiment is read inconjunction with the attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an X-ray diffraction of the precipitate obtained in Example 1(many phases).

FIG. 1B is an X-ray diffraction of the precipitate after dryingZnO+Zn(NH₃)₂Cl₂.

FIG. 1C is an X-ray diffraction of the precipitate after washing anddrying ZnO.

FIG. 2A is an X-ray diffraction of the precipitate obtained in Example 5(many phases).

FIG. 2B is an X-ray diffraction of the precipitate after dryingZnO+Zn(NH₃)₂Cl₂.

FIG. 2C is an X-ray diffraction of the precipitate after washing anddrying ZnO.

FIGS. 3 and 4 graphically illustrate the solubility characteristics ofzinc oxide in a sodium hydroxide solution.

FIG. 5 is a schematic diagram illustrating the general zinc oxidepurification process used in the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The method for recovering and purifying zinc oxide disclosed herein iscarried out in its best mode in recovering these materials from thewaste streams of industrial or other processes. A typical industrialwaste stream used is a flue dust where the charge contains galvanizedsteel, having the following percent composition:

TABLE I Analysis of Flue Dust Component Weight Percent zinc oxide 39.64iron oxide 36.74 lead oxide 5.72 inert materials 9.10 calcium oxide 2.80potassium oxide 2.41 manganese oxide 1.29 tin oxide 1.13 aluminum oxide0.38 magnesium oxide 0.33 chromium oxide 0.16 copper oxide 0.06 silver0.05 unidentified materials 0.22 TOTAL 100.00 1. siliceous material,such as slag, with carbon granules occluded 2. molybdenum, antimony,indium, cadmium, germanium, bismuth, titanium, nickel and boron

I. General Description of a Process for Producing a Zinc Oxide ProductSuitable for Purification by the Present Process

A waste material, typically a fly ash or flue dust such as EAF, isleached with an ammonium chloride solution resulting in a productsolution and undissolved materials. The product solution and theundissolved materials are separated, with both the product solution andthe undissolved materials being further treated to recover valuablecomponents. Zinc metal is added to the product solution to cement outany lead and cadmium contained in the product solution. The remainingproduct solution is rich in zinc compounds.

The remaining product solution then can be treated in two manners.First, the remaining product solution can be cooled therebyprecipitating the zinc components from the product solution as a mixtureof crystallized zinc compounds. These crystallized zinc compounds areseparated from the product solution, washed and then dried at elevatedtemperatures, resulting in a zinc oxide product of 99% or greaterpurity. Second, the remaining product solution can be subjected toelectrolysis in which zinc metal plates onto the cathode of theelectrolysis cell. Any remaining product solution after crystallizationor electrolysis is recycled back to treat incoming waste material. Thezinc oxide of 99% or greater purity is then further purified by apurification process which produces zinc oxide which is at least 99.8%pure. This purification process is preferably based on the solubility ofzinc oxide in a concentrated sodium hydroxide solution. However,intermediates other than sodium hydroxide can also be used. The resultis zinc oxide which is even of greater purity. Furthermore, the processcan be controlled to obtain zinc oxide having a desired size and surfacearea.

The undissolved material separated from the product solution is rich iniron oxide, and typically has some impurities such as zinc ferrite. Theundissolved materials can be used as a feedstock for steel mills so longas the quantity of impurities is not too great. It is preferable toremove the impurities from the iron oxide prior to using the iron oxideas a feedstock. Reducing the iron oxide to direct-reduced iron (DRI)also is desired as DRI can be used to replace part or all of the steelscrap charge.

The iron oxide in the undissolved materials can be reduced to DRI in twomanners. First, carbon, in the form of activated carbon, carbon dust,carbon pellets or the like, can be introduced to the ammonium chlorideand waste material mixture during the leaching process. The carbonreduces the iron oxide resulting in DRI. Second, the carbon can beintroduced to the dried undissolved material cake using a ribbonblender. The carbon will react with the iron oxide, reducing the ironoxide to DRI. Adding heat to this process assists in the reduction.

Prior to be leached by the ammonium chloride solution, the wastematerial, typically including franklinite and magnetite, may be roastedat temperatures greater than 500° C. for a predetermined period of time.The roasting causes a decomposition of the franklinite zinc oxide-ironoxide complex into zinc oxide, iron oxide and other components. Theroasting process generally comprises the steps of adding heat to thewaste material and/or passing heated reducing gases through the wastematerial. Although all reducing gases are suitable, hydrogen andcarbon-containing gases such as carbon dioxide are preferred, as well asmixing carbon (activated) with the material and roasting in a gascontaining oxygen. While some iron oxide is reduced from Fe₂O₃ and Fe₃O₄to FeO, no elemental iron is produced during the roasting step.Additionally, iron and iron oxides are not soluble to any degree in thebasic ammonium chloride solution.

The EAF dust can be heated in a reducing atmosphere to reduce theiron-zinc spinel into zinc oxide and iron oxide typically prior toleaching with ammonium chloride. An initial leach of the waste materialcan be done, followed by the roasting followed by another leach.Dispersants are used in the ammonium chloride solution to keep the zincpowder from clumping and thus increasing the efficiency of thecementation process. This method minimizes the formation of the diaminozinc dichloride, thus improving the washing step. In addition, theeffect of cooling profile on the particle size allows particle sizecontrol in the present process. This process also provides that the washwater stream must also be recycled as well as the steady stateconditions which will be achieved with the recycle.

In the leaching step, the zinc and/or zinc oxide dissolves in theammonium chloride solution along with other metal oxides contained inthe waste material, such as lead oxide and cadmium oxide. The resultantsolution is filtered to remove the undissolved materials, such as ironoxides and inert materials such as silicates, which will not dissolve inthe ammonium chloride solution. Finely powdered zinc metal can be addedto the resultant solution at a temperature of about 90° C. or above. Adispersant may be added at this point to prevent the finely powderedzinc metal from flocculating and becoming less effective. Through anelectrochemical reaction, lead metal and some cadmium plates out on thesurface of the zinc metal particles. The addition of sufficient powderedzinc metal results in the removal of virtually all of the lead from theresultant solution. The resultant solution is filtered to remove thesolid lead, zinc and cadmium. These initial steps, with the exception ofadding the dispersant, have been generally disclosed in the prior art,yet have not resulted in the production of essentially pure zinc oxide.

The filtrate then is cooled to a temperature of between about 20° C. and60° C. resulting in the crystallization of a mixture of zinc compounds.The crystallization step helps to achieve a high purity zinc oxide ofcontrolled particle size. During the crystallization step, the filtratecan be cooled to its final temperature by controlling the coolingprofile. The use of a reverse natural cooling profile is preferred asits results in a more desirable nucleation to crystal growth ratio. Thefiltrate contains a significant amount of diamino zinc dichloride, orother complex compounds which involve zinc amino complexes, as well ashydrated zinc oxide and hydroxide species. The solid precipitate isfiltered from the solution, the solution recycled, and the solidprecipitate washed with water at a temperature between about 25° C. and100° C. The diamino zinc dichloride dissolves in the wash water leavingthe majority of the hydrated zinc oxide species as the precipitatedsolid. The precipitated solid then is filtered from the solution, theresulting solution being recycled, and the solid precipitate placed in adrying oven at a temperature above 100° C. and preferably between about100° C. and 350° C., resulting in a dry white zinc oxide powder. Theseadditional steps allow the production and recovery of substantially purezinc oxide. Alternatively, the filtrate can be subjected to electrolysisto recover zinc metal.

Generally, the zinc oxide production process comprises the steps of:

a. roasting the waste material at an elevated temperature and in areducing atmosphere;

b. treating the waste material with an ammonium chloride solution at anelevated temperature to form a product solution which comprisesdissolved zinc and dissolved zinc oxide whereby any iron oxide in thewaste material will not go into solution;

c. separating the product solution from any undissolved materialspresent in the product solution including any of the iron oxide;

d. adding zinc metal and a dispersant to the product solution wherebyany lead and cadmium ions contained within the product solution aredisplaced by the zinc metal and precipitate out of the product solutionas lead and cadmium metals and the dispersant is selected from the groupconsisting of dispersants which will prevent the aggregation of saidzinc metal;

e. separating the product solution from the lead and cadmium metals;

f. lowering the temperature of the product solution therebyprecipitating the zinc component as a mixture of crystallized zinccompounds;

g. separating the precipitated zinc compounds from the product solution;

h. washing the zinc compounds solids with a wash water therebysolubilizing certain of the zinc compounds;

i. separating the remaining zinc compounds solids from the solution; andthen

j. drying the remaining zinc compounds solids at a temperature of atleast 100° C. whereby the resulting product is zinc oxide of 99% orgreater purity.

The process also can comprise a two-stage leaching process for evengreater yields of zinc oxide. The two-stage process comprises the stepsof:

a. treating the waste material a first time with an ammonium chloridesolution at an elevated temperature to form a first product solutionwhich comprises dissolved zinc constituents whereby any iron oxide inthe waste material will not go into solution;

b. separating the first product solution from the undissolved wastematerial compounds present in the first product solution including anyof the iron oxide;

c. roasting the undissolved waste material compounds at an elevatedtemperature and in a reducing atmosphere;

d. treating the roasted undissolved waste material compounds a secondtime with the ammonium chloride solution at an elevated temperature toform a second product solution which comprises dissolved zincconstituents whereby any iron oxide remaining in the roasted undissolvedwaste material compounds will not go into solution;

e. combining the first and second product solutions to form a combinedproduct solution, maintaining the combined product solution at atemperature of at least 90° C., and adding powdered zinc metal and adispersant to the combined product solution whereby any lead and cadmiumions contained within the combined product solution are displaced by thezinc metal and precipitate out of the combined product solution as leadand cadmium metals and the dispersant is selected from the groupconsisting of dispersants which will prevent the aggregation of the zincmetal; and

f. separating the combined product solution from the lead and cadmiummetals.

After the combined product solution is separated from the lead andcadmium metals, the combined product solution is treated similarly tothe treatment of the product solution in steps f through j of thegeneral method disclosed above.

An ammonium chloride solution in water is prepared in known quantitiesand concentrations. If the two-stage leaching process is used, the feedmaterial which contains the zinc species, such as the waste materialflue dust described in Table I or any other feed material source whichcontains zinc or zinc oxide mixed with other metals, is added to theammonium chloride solution at a temperature of about 90° C. or above.Otherwise, the feed material is roasted. The zinc and/or zinc oxidedissolves in the ammonium chloride solution along with other metaloxides, such as lead oxide and cadmium oxide. The iron oxide does notdissolve in the ammonium chloride solution. The solubility of zinc oxidein ammonium chloride solutions is shown in Table II.

TABLE II Solubility of ZnO in 23% NH₄Cl solution Temperature ° C. gDissolved/100 g H₂O 90 14.6 80 13.3 70  8.4 60  5.0 50  3.7 40  2.3

A 23% by weight ammonium chloride solution in water at a temperature ofat least 90° C. provides the best solubility of zinc oxide.Concentrations of ammonium chloride below about 23% do not dissolve themaximum amount of zinc oxide from the flue dust, and concentrations ofammonium chloride above about 23% tend to precipitate out ammoniumchloride along with the zinc oxide when the solution is cooled. Ironoxide and inert materials such as silicates will not dissolve in thepreferred solution.

The zinc oxide, as well as smaller concentrations of lead or cadmiumoxide, are removed from the initial dust by the dissolution in theammonium chloride solution. The solid remaining after this leaching stepcontains zinc, iron, lead and cadmium, and possibly some otherimpurities. The remaining solid then is roasted in a reducingatmosphere, typically at a temperature greater than 420° C. and often at700° C. to 900° C. The reducing atmosphere can be created by usinghydrogen gas, simple carbon species gases such as carbon dioxide, or byheating the material in an oxygen containing gas in the presence ofelemental carbon. The carbon preferably is in the form of dust orpellets. Typical roasting times are from 30 minutes to 4 hours. Asdiscussed above, the waste dust first may be roasted and second may beleached, omitting the first leaching step.

After the dust has been roasted, it is subjected to a leaching step in23% by weight ammonium chloride solution in water at a temperature of atleast 90° C. Any zinc or zinc oxide formed during the roasting stepdissolves in the ammonium chloride solution. The zinc oxide and ammoniumchloride solution then is filtered to remove any undissolved material,including the iron oxide. After filtering, for analysis, the solid maybe separated out and dried at a temperature of over 100° C., typicallybetween 100° C. and 200° C., for about 30 minutes to 2 hours, typicallyapproximately 1 hour.

To recover the zinc oxide, while the filtered zinc oxide and ammoniumchloride solution is still hot, that is at a temperature of 90° C. orabove, finely powdered zinc metal is added to the solution. Through anelectrochemical reaction, any lead metal and cadmium in solution platesout onto the surfaces of the zinc metal particles. The addition ofsufficient powdered zinc metal results in the removal of virtually allof the lead of the solution. The solution then is filtered to remove thesolid lead, zinc and cadmium.

Powdered zinc metal alone may be added to the zinc oxide and ammoniumchloride solution in order to remove the solid lead and cadmium.However, the zinc powder typically aggregates to form large clumps inthe solution which sink to the bottom of the vessel. Rapid agitationtypically will not prevent this aggregation from occurring. To keep thezinc powder suspended in the zinc oxide and ammonium chloride solution,any one of a number of water soluble polymers which act asantiflocculants or dispersants may be used. In addition, a number ofsurface active materials also will act to keep the zinc powdersuspended, as will many compounds used in scale control. These materialsonly need be present in concentrations of 10-1000 ppm. Various suitablematerials include water soluble polymer dispersants, scale controllers,and surfactants, such as lignosulfonates, polyphosphates, polyacrylates,polymethacrylates, maleic anhydride copolymers, polymaleic anhydride,phosphate esters and phosphonates. A discussion of these variousmaterials can be found in the literature, such as Drew, Principles ofIndustrial Waste Treatment, pages 79-84, which is incorporated herein byreference. Flocon 100 and other members of the Flocon series ofmaleic-based acrylic oligomers of various molecular weights of watersoluble polymers, produced by FMC Corporation, also are effective.Adding the dispersants to a very high ionic strength solution containinga wide variety of ionic species is anathema to standard practice asdispersants often are not soluble in such high ionic strength solutions.

At this stage there is a filtrate rich in zinc compounds and aprecipitate of lead, cadmium and other products. The filtrate andprecipitate are separated, with the precipitate being further treated,if desired, to capture chemical values. The filtrate may be treated inseveral manners, two of which are preferred. First, the filtrate may becooled resulting in the crystallization and recovery of zinc oxide.Second, the filtrate may be subjected to electrolysis resulting in thegeneration and recovery of metallic zinc.

To recover zinc oxide, the filtrate then is cooled to a temperature ofbetween about 20° C. and 60° C. resulting in the crystallization of amixture of zinc compounds. The mixture contains a significant amount ofdiamino zinc dichloride, or other complex compounds which involves zincamino complexes, hydrated zinc oxides and hydroxide species.Crystallization helps to achieve a high purity zinc oxide of controlledparticle size, typically through control of the temperature-time coolingprofile. Reverse natural cooling, that is cooling the solution slower atthe beginning of the cooling period and faster at the end of the coolingperiod, is preferred to control the nucleation to crystal growth ratioand, ultimately, the crystal size distribution. The precipitatedcrystallized solid is filtered from the solution and washed with waterat a temperature of between about 25° C. and 100° C. The filteredsolution is recycled for further charging with feed material. Thediamino zinc dichloride dissolves in water. The solubility of diaminozinc dichloride in water is shown in Table III.

The amount and temperature of the wash water can be selected to achievea particular size/surface area of the zinc oxide crystals. We have foundthat at a fixed temperature, the more wash water used, the larger thesurface area (smaller particles) and at a fixed amount of wash water,the higher the temperature, the larger the surface area (smallerparticles). Any temperature above 25° C. may be used in this process tocontrol the particle characteristics. Temperatures above 100° C. may beused if done under pressure. However, because of the added expense ofheating and maintaining a temperature above 100° C., and because ZnOHforms at a greater rate as the temperature decreases below 60° C., it ispreferred to use a temperature of 60-100° C. Through variations in thetwo conditions, using the method disclosed herein, one skilled in theart can produce zinc crystals with a predetermined particle size, andthus, surface area.

TABLE III Solubility of Zn(NH₃)₂Cl₂ in water Temperature ° C. gDissolved/100 g H₂O 90 32   80 24   40 21   25 12.8

Very little of the hydrated zinc oxide dissolves in the water. Thisresultant solution then is filtered to remove the hydrated zinc oxidespecies. The solid hydrated zinc oxide species filtered from thesolution is placed in a drying oven at a temperature of over 100° C.After a sufficient drying period, the resultant dry white powder isessentially pure zinc oxide. The filtrate from the solution is recycledfor charging with additional zinc compound mixture.

The zinc oxide may be dried at approximately 100° C. To ensure that thematerial is free of chloride, however, it is preferable to heat the zincoxide to a higher temperature. Diamino zinc dichloride decomposes at271° C. and ammonium chloride sublimes at 340° C. Therefore, heating thezinc oxide to a temperature above 271° C. is useful. The dryingtemperature should be kept below approximately 350° C. to prevent thesublimation of significant amount of ammonium chloride. Therefore, it ispreferable to dry the zinc oxide at a temperature in the range of 271°C. to 350° C. Typically, the zinc oxide should be dried in thistemperature range for approximately 2 to 60 minutes, and preferably from5 to 20 minutes. A 10 minute drying time has been found to be asatisfactory average.

As the zinc, lead and cadmium contained in the feed materials areamphoteric species, by using ammonium chloride solution these specieswill go into solution, while any iron oxide present in the feed materialwill not go into solution. Other solutions, such as strong basicsolutions having a pH greater than about 10 or strong acidic solutionshaving a pH less than about 3, also can be used to dissolve the zinc,lead and cadmium species; however, if strong acidic solutions are used,iron oxide will dissolve into the solution, and if strong basicsolutions are used, iron oxide will become gelatinous. The lead andcadmium can be removed from the ammonium chloride solution through anelectrochemical reaction which results in the precipitation of lead andcadmium in elemental form. The difference in solubility between diaminozinc dichloride and zinc oxide in water and in ammonium chloridesolutions allows the selective dissolution of the diamino zincdichloride such that pure zinc oxide can be recovered. This also can beused in the crystallization step to improve the relative amounts ofdiamino zinc dichloride and zinc oxide species form. Significantly, allof the zinc can be recycled so that all of the zinc eventually will beconverted into zinc oxide.

The crystallization step can be done continuously in order to increasethe throughput and maximize the zinc oxide yield after the washing anddrying step.

The following Examples demonstrate ways to increase the formation ofzinc oxide according to this process. Examples 1-7 do not includeroasting and Examples 8-13 include roasting. Examples 10-12 also showvariations on the crystallization step, and Example 13 also illustratesthe recycle results. X-ray diffraction analyses of the zinc oxideprepared according to these examples indicate the recovery of highpurity zinc oxide.

EXAMPLE 1 Prior Art

A metal dust of composition listed in Table I of the Burrows patent isadded to 23% by weight NH₄Cl solution (30 g NH₄Cl per 100 g H₂O), asdiscussed in the Burrows patent, in the amount of 1 gram of dust per 10grams of solution. The solution is heated to a temperature of 90° C. andstirred for a period of 1 hour, during which the zinc oxide in the dustdissolves. The remaining solid, which has a composition of approximately60% iron oxide, 5% calcium oxide, 5% manganese, 30% other materials, isfiltered out of the solution. Powdered zinc then is added to thefiltrate at 90° C., causing the precipitation of waste metals, theprecipitate containing about 60% lead, 40% zinc, 2% cadmium and 8% othermetals. The waste metals then are filtered out and the filtrate iscooled to room temperature (between about 18° C. and 30° C.) over aperiod of about two hours. The solution then contains a whiteprecipitate which is not essentially pure zinc oxide but is a mixture ofhydrated zinc phases and diamino zinc dichloride.

EXAMPLE 2

A metal dust of composition listed in Table I is added to 23% weightNH₄Cl solution (30 g NH₄Cl per 100 g H₂O). 1 gram of dust is used per 10grams of solution. The solution is heated to a temperature of 90° C. andstirred for a period of 1 hour. During this period the zinc oxide in thedust dissolves. The remaining solid, having a composition ofapproximately 60% iron oxide, 5% calcium oxide, 5% manganese, 30% othermaterials, is filtered out of the solution. Powdered zinc then is addedto the filtrate at 90° C. This causes the precipitation of waste metals,the waste metal precipitate containing about 60% lead, 40% zinc, 2%cadmium and 8% other metals. The waste metals then are filtered out andthe filtrate is cooled to room temperature (between about 18° C. and 30°C.) over a period of about two hours. The solution then contains a whiteprecipitate.

As shown in FIG. 1A, X-ray diffraction of the precipitate indicates thatit is a mixture of hydrated zinc phases and diamino zinc dichloride. Thehydrated zinc phases are virtually insoluble in water; however, themeasurements in Table III show that diamino zinc dichloride is quitesoluble in water. A portion of the white precipitate was dried and, asshown in FIG. 1B, zinc oxide and diamino zinc dichloride, as well assome other components, are present. The white precipitate then isfiltered from the solution and resuspended in water at 90° C. andstirred for a period of one hour. This suspension then is filtered andproduct dried in an oven at 140° C. As shown in FIG. 1C, the resultingwhite solid is 99%+zinc oxide. The amount of zinc oxide obtained was47.8% of the mass of the original precipitate.

The ZnO recovered by this Example also had the following components:

lead: 866 ppm potassium: 45 ppm calcium: <25 ppm manganese: <25 ppmchromium: <25 ppm

EXAMPLE 3

The procedure of Example 1 is followed until the step in which the zinccontaining filtrate is cooled. Since the diamino zinc dichloride is moresoluble then the majority of the other possible precipitates in theammonium chloride solution (except for zinc chloride which is so solublethat it will not appear), the diamino zinc dichloride appears as alarger fraction of the solid as the temperature declines. The filtratewas divided into fractions and each fraction cooled to a differenttemperature. The resulting solids were than filtered, resuspended inwater at 90° C. for one hour, filtered and dried. The result was99%+zinc oxide in all cases; however, the yield changed with thetemperature to which the fraction was cooled as follows:

Crystallization Percent ZnO Temp (° C.) Obtained 75 65 70 60 60 60 50 50

Crystallization at temperatures from 60° C. up improve the yield of ZnO.

EXAMPLE 4

ZnO also can be recovered from the wash water used in the process. Fiftygrams of dried zinc phase precipitate (the solid obtained after coolingto room temperature) obtained using the procedure of Example 1 is addedto 100 g of H₂O at 90° C. The diamino zinc dichloride dissolves whileonly a small amount of the other zinc phases dissolve (due to theammonium chloride which is part of the diamino zinc dichloride). Theremaining solid is filtered out and is dried resulting in 99%+zincoxide. The filtrate is cooled to room temperature and the solid filteredout. The solid is again a mixture of hydrated zinc phases andZn(NH₃)₂Cl₂. The solid is washed in 90° C. water, filtered and driedresulting in 99% ZnO. The yield is 40% ZnO.

The yield also can be improved by crystallizing at higher temperatures.In addition, the same wash water can be used again instead of freshwater since this part of the process relies on the change in Zn(NH₃)₂solubility with temperature.

EXAMPLE 5

The source of the zinc does not have to be dust. If pure ZnO is added toa 23% NH₄Cl solution, the result is the same. As an example, saturatedsolutions of ZnO in 23% ammonium chloride solutions were prepared attemperatures ranging from 40° C.-90° C., using the solubility data ofTable II. These solutions were then cooled to room temperature over aperiod of 1-2 hours. The resulting solid was filtered, washed in 90° C.water, and dried. As before, and as shown in FIG. 2A, the original solidwas a mixture of hydrated zinc phases and diamino zinc dichloride. Asshown in FIG. 2C, the final product was 99% ZnO. FIG. 2B shows theanalysis of the intermediate zinc oxide and diamino zinc dichlorideprecipitate. The yields obtained as a fraction of the original solidprecipitate are listed below:

Temperature ZnO ZnO Obtained (° C.) Added (g) in Product (%) 90 14.6 6480 13.2 62 70  8.4 60 60  5.0 60 50  3.7 45 40  2.3 40

These results indicate that the yield of ZnO improves as the amount ofdissolved ZnO increases (which also means higher temperatures).

EXAMPLE 6

This example shows the process run in a continuous crystallizationprocess to increase the throughput and to maximize the zinc oxide yield.The procedure of Example 1 is followed until the step in which the wastemetals are precipitated out of the zinc oxide containing solution. Fiftygallons of the solution are used as the feedstock for a continuouscrystallization process. The solution, initially at about 90° C., ispumped into a 1-gallon jacketed crystallizer equipped with baffles and adraft tube at a rate of 1 gallon per hour. The crystallizer jackettemperature is maintained at about 55° C. by use of a constanttemperature circulating bath. The solution and the product crystals areremoved continuously so as to keep the volume of material present in thecrystallizer constant. At steady state, the temperature in thecrystallizer is maintained at about 60° C. The product solution flowsthrough a filter which collects the solid. The solid product thenundergoes the washing and drying steps as discussed in Example 2. Theyield of zinc oxide from this continuous crystallization process isabout 60% of the total mass of the solid crystallized.

The crystallizer can be operated at lower temperatures; however, lowertemperatures decrease the final yield of zinc oxide obtained as shown inExample 3. The flow rate employed also can be altered along with thecrystallizer jacket temperature to minimize crystallization on thecrystallizer vessel walls. In addition, these variables, along with thecrystallizer jacket temperature, can be used to alter the crystal sizedistribution.

EXAMPLE 7

Metal dust of the composition shown in Table I is digested in 23%ammonium chloride solution at about 90° C. One gram of zinc metal dustis used per 10 grams of ammonium chloride solution. After one hour, theremaining solid is filtered out of the solution. 500 cc of the solutionis put into each of two vessels with stirrers and the temperature of thesolutions is maintained at 90° C. 500 ppm of Flocon 100 is added to oneof the vessels, while nothing is added to the other vessel. Four-tenthsof a gram (0.4 g) of 200 mesh zinc dust then is added to each of the twosolutions. In the solution containing the Flocon 100, the zinc dustremains suspended, while in the other solution containing no additives,the zinc dust clumps together (flocculates). After one hour at about 90°C., the solids are filtered out of each of the solutions, weighed andanalyzed. The mass of solid from the solution which contained thedispersant was 1.9 grams and comprised approximately 21% zinc, 75% lead,2% cadmium and the remaining amount other metals. The mass of solidobtained from the solution with no dispersant was 1.2 grams andcomprised approximately 33% zinc, 63% lead, 2% cadmium and the remainingamount other metals. From this example, it can be seen that theadditional step of adding a dispersant increases the amount of lead andother metals removed from the waste stream in solution.

EXAMPLE 8

Crude crystals of diamino zinc dichloride were obtained using theprocedure of Example 1. These crystals were washed with water at ratiosof 0.1 lb. per gallon, 1.0 lb. per gallon and 2.0 lb. per gallon attemperatures of 80° C., 90° C. and 100° C. for a period of 5 minutes.The resulting crystals were dried and the surface area measured usingthe nitrogen BET method.

Surface Area (m²/g) In Relation To Temperature (° C.) And Amount OfWater (lb./gallon) 80° C. 90° C. 100° C. 0.1 lb./gallon 13.37 18.8928.17 1.0 lb./gallon  1.56  2.30  8.90 2.0 lb./gallon  0.84  1.50  6.07

This example indicates that surface area can be controlled by both theuse of temperature and dilution. It would also be possible to usetemperature above 100° C. under pressure to achieve similar results.This principle can be employed to design washing conditions to producezinc oxides of a desired average size and distribution.

A. Roasting Step for Enhanced Zinc Recovery

The zinc dust obtained from various sources have shown by chemicalanalysis to contain from 20%-25% zinc by weight. X-ray diffractionindicates clearly the existence of certain crystalline phases in thisdust, specifically zinc oxide. The positive identification of the ironphase is complicated by the possible structural types (i.e. spinel typeiron phases showing almost identical diffraction patterns). The zincoxide (as well as smaller concentrations of lead or cadmium oxide) areremoved from the initial dust by dissolution in a concentrated ammoniumchloride solution (23% ammonium chloride).

Filtration and washing of the undissolved species leaves a residualpowder. This powder shows a zinc concentration that is still elevated(i.e., 10-13% by weight), but that is not zinc oxide. X-ray diffractionindicates that all crystalline phases can be identified by spinel typephases. The combination of chemical analysis and x-ray diffractionindicates that this powder is a combination of magnetite (iron oxide:Fe₃O₄). Both of these phases have very similar spinel type structures.The zinc within the franklinite, (Fe, Mn, Zn)(FeMn)₂O₄, cannot beremoved by dissolution with ammonium chloride. In addition, no simpleextraction process will remove zinc from this stable oxide phase.Although this compound is very stable to oxidation (all elements in thehighest oxidation state), it is relatively easy to destroy this compoundby reduction at elevated temperatures. The reduction of the franklinitein an atmosphere that cannot readily reduce zinc oxide or allow for therapid oxidation of zinc to zinc oxide following reduction andsubsequently recover the zinc oxide by ammonium chloride extraction orsublimation (the highly volatile zinc oxide will sublime from themixture at relatively low temperatures and recondense at the coldlocations of the roaster). The alternative will be complete reduction ofthe franklinite to zinc metal and removal by distillation or separationof the molten zinc by settling techniques.

1. Roasting Process:

The roasting step can be carried out prior to the initial leaching step,or between a first and second leaching step. The powder containing thefranklinite and magnetite, such as the waste dust, is heated totemperatures greater than 500° C. This temperature causes a reactionwhich causes a decomposition of the stable franklinite phase into zincoxide and other components, and yet does not allow for the completereduction of zinc oxide to zinc metal. The resulting zinc oxide can beremoved by sublimation or extraction with an ammonium chloride solution,such as by following the steps detailed above under the general process.The resulting material after extraction has less than 1% by weight zinc.

The dust can be roasted using many conventional roasting processes, suchas, for example, direct or indirect heating and the passing of hot gasesthrough the dust. For example, non-explosive mixtures of reducing gases,such as for example hydrogen gas and nitrogen or carbon dioxide, can bepassed through the powder containing franklinite and magnetite. Hydrogengas is not the only species that may be used for reductive decompositionof franklinite. It is possible to use carbon or simple carbon containingspecies, including carbon-containing reducing gases and elementalcarbon. Heterogeneous gas phase reductions are faster than solid statereductions at lower temperatures and therefore suggest the use of carbonmonoxide. The carbon monoxide can be generated in situ by mixing thefranklinite powder with carbon and heating in the presence of oxygen atelevated temperatures. The oxygen concentration is controlled tooptimize CO production. The carbon monoxide may be introduced as aseparate source to more clearly separate the rate of carbon monoxidepreparation from the rate of Franklinite decomposition. The preparedzinc oxide then can be removed by either ammonium chloride extraction orsublimation.

The roasting process also can be performed to complete reduction byusing carbon at high temperatures and collecting zinc metal that willmelt at very low temperatures (420° C.) and boil at 907° C. In thisprocess, zinc metal is obtained that, if desired, can be convertedreadily to the oxide by air roasting.

EXAMPLE 9

A dust containing 19.63% Zn, 27.75% Fe, 1.31% Pb, 9.99% Ca, and 0.024%Cd (analysis based on elements not oxides) was leached at 100° C. in a23% ammonium chloride solution. The solid remaining after the leachingprocess was dried and analyzed to contain 12.67% Zn, 4.6% Ca, 35.23% Fe,0.7% Pb, and 0.01% Cd. This material was placed in a quartz boat in thepresence of activated carbon and heated at 900° C. for two hours in anatmosphere of 95% N₂ and 5% O₂. After two hours, the material wasremoved and added to a 23% ammonium chloride solution at 100° C. Thematerial was filtered and dried at 140° C. for one hour to determine itscomposition. Analysis of this remaining solid was 42.84% Fe, 0.28% Zn,<0.1% Pb, and <0.01% Cd. The leached-roasted-leached material then canbe subjected to the remainder of the general process to recover zincoxide.

EXAMPLE 10

A dust with composition given in Table I is leached in 23% ammoniumchloride solution for 1 hour at 100° C. The solid remaining (whichcontained 14% Zn) was placed in a quartz boat and heated to 700° C. inan atmosphere of 8% H₂ and 92% Ar. The material was cooled and reheatedat 100° C. in 23% ammonium chloride solution at 100° C. The solid wasseparated, dried and analyzed for zinc. The zinc was found to be lessthan 1%. The leached-roasted-leached material then can be subjected tothe remainder of the general process to recover zinc oxide.

2. Crystallization Step Variations:

The purpose of the crystallization/washing step is to produce a highpurity zinc oxide of controlled particle size. This is accomplishedthrough control of the temperature-time profile during cooling in thecrystallization.

The crystallization step in the process takes the filtrate from thecementation step at 90-100° C. This filtrate contains the dissolved zincwith small amounts of trace impurities such as lead and cadmium. Inorder to prepare a pure zinc oxide it is necessary to prevent theformation of solvent inclusions inside the grown crystals. Solventinclusions are pockets of liquid trapped as a second phase inside thecrystals. Control of crystallization conditions can be employed toreduce these impurities. An example is given below.

EXAMPLE 11

A dust of the composition given in Table I is taken through the leachingand cementation steps. After cementation the filtrate is maintained at100° C. 500 ml of this filtrate is placed in a jacketed stirred vesselwith the jacket temperature at 100° C. The temperature is lowered in thecrystallizer as follows:

Time (minutes) Temperature (° C.)  0 100  60  90 120  75 180  55 210  25

The resulting solid was washed and dried employing the proceduredescribed above. The resulting material was analyzed as follows:

ZnO 99 + % Lead <50 ppm Cd <25 ppm Fe <25 ppm

The cooling profile in Example 11 is known as a reverse natural coolingprofile. Such a profile is the opposite shape as that which is observedby natural cooling. In a reverse natural cooling profile, the cooling isslower at the beginning and faster at the end; in a natural coolingprofile, the cooling is faster at the beginning and slower at the end.This type of cooling profile also is used to control the crystal sizedistribution (CSD) of the zinc oxide obtained. The cooling profilecontrols the ratio of nucleation (birth of a new crystal) to crystalgrowth (growth of existing crystals). The ratio of nucleation/growthdetermines the final CSD.

EXAMPLE 12

A 23% ammonium chloride solution at 100° C. containing 11% by weightdissolved ZnO is divided into 4 portions. Each portion is placed in ajacketed agitated vessel. The cooling profiles in each vessel are givenbelow:

Time (minutes) Temp. (° C.) Time (minutes) Temp. (° C.) Vessel A VesselB  0 100  0 100  60 75  60 50 120 50 120 37.5 180 25 180 25 Vessel CVessel D  0 100  0 100  60 87.5  60 87.5 120 75 120 75 180 25 180 62.5270 25

The solid is washed using the usual procedures described previously. Theaverage size and size distribution of these materials were measuredusing a laser light scattering particle size analyzer. The results wereas shown below:

Vessel Mean Size A 22 B 19 C 27 D 37

The results show that controlling the temperature with a reverse naturalcooling curve results in a larger average size than by linear cooling(A) or natural cooling (B). This principle can be employed to designcooling profiles to produce zinc oxides of a desired average size anddistribution.

3. Recycle:

To produce pure zinc oxide from waste dust containing zinc efficientlyand in a safe and cost effective way, the process recycles all zincwhich is not removed from the leachate in the crystallization step. Inaddition, the diamino zinc dichloride which is redissolved in water inthe washing step also is recycled. The recycle of zinc increases theoverall zinc concentration in liquid solution in the process. Thisallows the crystallizer to operate at a higher temperature due to therapid change in zinc oxide solubility with temperature in ammoniumchloride solution. An example of the process with recycle is givenbelow:

EXAMPLE 13

By controlling the recycle, the steady state zinc concentration can beraised to 7 g/100 g of solution. If the outlet of the crystallizer iskept at 60° C., 3 g/100 g solution of solid will crystallize (the solidis a mixture of zinc oxide and diamino zinc dichloride). The system doesnot have to be cooled further since this is an efficient way to operateto conserve energy (one does not have to cool then reheat the solution).In addition, operating at the higher Zn concentration improves the ratioof ZnO/diamino zinc dichloride produced in the crystallizer.

The recycle has the advantage that the solution become saturatedrelative to certain materials present in the dust, such as CaO. Whenthis occurs, CaO no longer is leached from the dust but remains with theiron in the iron cake. This increases the value of the cake since CaO isstill present and will not have to be added when the iron cake is fed toa furnace in steel making. Another important advantage in that there isno liquid effluent in this process. The only products are solid (ironcake, zinc oxide, waste metals), which are then sold for use in variousindustrial processes. No waste is produced since all liquid is recycled.

B. Carbon Addition Step for Recovery of Iron-Carbon and Direct-ReducedIron Products

The process also can be operated to produce a high-quality iron-carboncake as a residual product. The iron oxide contained in the waste streamdoes not go into solution in the ammonium chloride solution, but isfiltered from the product solution as undissolved material. This ironoxide cake can be used as is as the feedstock to a steel mill; however,it becomes more valuable if reduced by reaction with elemental carbon toproduce an iron-carbon or direct-reduced iron product. One preferredmethod for producing such an iron-carbon or direct-reduced iron productfrom the waste material comprises the steps of:

a. treating the waste material with an ammonium chloride solution at anelevated temperature to form a product solution which comprisesdissolved zinc and dissolved zinc oxide whereby any iron oxide in thewaste material will not go into solution;

b. adding carbon to the product solution whereby the carbon will not gointo solution; and then

c. separating the product solution from any undissolved materialspresent in the product solution including any of the iron oxide and thecarbon.

A mixture of iron oxide and carbon is used by the steel industry as afeedstock for electric arc furnaces. The iron oxide cake which isremoved as undissolved material from the leaching step is primarily ironoxide, being a mixture of Fe₂O₃ and Fe₃O₄. The iron cake can be madeinto the mixture of iron oxide and carbon by adding elemental carbon tothe iron oxide cake in several manners. First, carbon can be added tothe leaching tank at the end of the leaching step but before theundissolved materials are separated from the product solution. Since thecarbon is not soluble in the ammonium chloride solution and will notreact in an aqueous solution, the iron cake and the carbon can beseparated from the product solution and made into a hard cake. Differentsize carbon, such as dust, granules, or pellets, may be used dependingon the desires of the steel makers. Second, the carbon can be added tothe iron oxide after the iron oxide has been separated from the productsolution. The dried iron oxide and the carbon can be ribbon blended in aseparate process.

Combining carbon and iron oxide results in the reduction of the ironoxide, producing direct-reduced iron (DRI). DRI can be used to replacepart or all of the steel scrap charged to a steel mill. In someoperations, DRI is preferred to scrap because it has a known uniformcomposition and generally contains no residual elements such aschromium, copper, nickel, and tin. Also, when DRI is melted, it forms afoamy slag because it contains both carbon and iron oxide. Because theprice of steel scrap usually is lower than DRI, the use of DRI usuallycannot be economically justified. DRI typically runs in the $120.00 perton range. However, since the iron oxide is a residual product of thisprocess, with the main value of the process being from the zinc oxideproduct, the iron oxide or direct-reduced iron can be produced moreeconomically. Therefore, the iron oxide produced as a residual in thisprocess has significant value.

The undissolved materials primarily comprise iron oxide and carbon whichhas significant value as a feedstock to a steel mill, as discussedabove. Generally the iron oxide and carbon product is pressed into acake for ease of handling and use. The cake typically containsapproximately 82% solids, but may range from 78% to 86% solids and beeasily handled and used. Although cakes of less than 78% solids can beformed, the other 22%+of material would be product solution which, ifthe cake is used as a feedstock to a steel mill, would be reintroducedto the steel-making process, which is uneconomical. Likewise, drying thecake to have more than 86% solids can be uneconomical. The productsolution from this process can be treated similarly to the treatment ofthe product solution in steps d through j of the general methoddisclosed above.

The roasting process produces vapors, from the zinc, lead and cadmiumand other impurities, that have to be condensed into dust. Theseimpurities can be sent to the baghouse at the end of the steel makingprocess, mixed into the original waste dust, and then sent to the firstleaching step, in a recycle fashion. Alternatively, the exhaust vaporsand dust from the roasting step may be sent to a separate baghouse at astand alone facility.

There are two preferred ways to add carbon to the iron oxide cake.First, it may be beneficial when the iron oxide cake comes out of thereclamation process to grind up the iron oxide cake, pelletize it withcarbon and put it in a roasting furnace. Second, carbon can be added tothe furnace with the iron oxide.

The iron oxide cake can be treated in three manners. First, and leastpreferable, carbon can be added to the leaching step and the iron oxidecake will have carbon plus iron oxide. The iron oxide-carbon cake can godirectly to the steel mill and, if it goes directly to the steel mill,then the reduction of the iron oxide would take place in the steel millfurnace. Second, and most preferable, the iron oxide-carbon cake can bepelletized and roasted in a reduction furnace to form direct reducediron. The iron oxide precipitate, which typically contains around 80%solids, is ground up with carbon and formed into pellets, briquettes orcubes and then heated. These pellets, briquettes or cubes then can beintroduced to a steel making furnace. The difference in the materialthat would be introduced to the furnace from the first manner and thesecond manner is that in the second manner, direct reduced iron isintroduced to the steel making furnace, while in the first manner, acombination of iron oxide and carbon is introduced to the steel makingfurnace. The iron oxide plus carbon can be supplied to the steel mill asis. When this carbon enriched iron oxide is melted, it forms a foamyslag, and a foamy slag is desirable in steel making. Third, the carboncan be added through a ribbon blender, and then the iron oxide-carboncake can be introduced either directly into the furnace or, preferablyroasted in a reduction furnace first to form direct reduced iron, whichwould be preferred for steel making.

In any manner, the fumes exhausting from the steel mill furnace and thereduction furnace typically are iron poor, but comprise other valuablecomponents. The furnace exhaust fumes are an excellent source of ironpoor waste materials useful for recovery in the present process. Theexhaust fumes may be filtered in a baghouse, with the resulting filtratebeing added to the waste stream feed of the present process, or with theresulting filtrate being the primary waste stream feed of the presentprocess. The exhaust fumes also may be scrubbed in a wet scrubber, withthe resulting loaded scrubbing solution being added to the ammoniumchloride leachant of the present process. If an ammonium chloridescrubbing solution is used instead of water, the loaded ammoniumchloride scrubbing solution may be used as the primary leachant of thepresent process.

C. Iron By-Product Recycle

Iron-rich by-products produced during the recovery process can beprocessed further to obtain an end product which can be recycled backinto the leaching step of the recovery process of the present invention.The iron-rich by-products preferably are reduced to DRI in a reductionfurnace. During the reduction process, exhausts fumes which consistsprimarily of zinc, lead and cadmium are produced in the reductionfurnace.

In accordance with a first embodiment, the DRI is sent to a steel millwhere it is used in the production of steel. The steel productionprocess results in exhaust fumes which are processed through thebaghouse or/and a wet scrubber, either or both of which can be locatedat the steel mill. Fumes processed through the baghouse are filtered,and the captured solid residuum, along with an added amount of EAF dust,is recycled back into the waste materials stream whereby it is returnedto the leaching step of the recovery process. Fumes processed throughthe wet scrubber are scrubbed in a liquid stream and the residualimpurities obtained from the scrubbing process are discharged from thewet scrubber directly into the ammonium chloride solution of theleaching step.

In accordance with a second embodiment, the fumes exhausted from thereduction furnace used to produce the DRI are processed through thebaghouse or/and the wet scrubber. Fumes processed through the baghouseare filtered, and the captured solid residuum is recycled back into thewaste material stream, whereby it is returned to the ammonium chloridesolution of the leaching step. In this embodiment, no EAF dust need beadded in with the solid residuum. Fumes processed through the wetscrubber are scrubbed in a liquid stream and the residual impuritiesobtained from the filtering process are discharged from the wet scrubberdirectly into the ammonium chloride solution of the leaching step.

Therefore, iron-rich products which are produced during the recoveryprocess of the present invention can be further processed to producefumes consisting primarily of zinc, lead and cadmium which are capturedin a baghouse or/and a wet scrubber and recycled back into the ammoniumchloride solution of the leaching step to be used in the recoveryprocess.

It should be noted that the locations of the baghouse and wet scrubberare a matter of design choice, plant efficiency and convenience. Thepresent invention is not limited in this aspect. For example, steelmills are equipped with baghouses and wet scrubbers which can be used inthe recycling process of the present invention. Similarly, the locationsof the baghouse or wet scrubber used to process fumes from the DRIreduction furnace are also a matter of design choice, plant efficiencyand convenience.

D. Electrolysis Step for Zinc Recovery

The process can be operated to recover zinc metal by replacing thecrystallization steps with an electrolysis step. One preferred methodfor the recovery of zinc oxide from waste material streams whichcomprise zinc compounds using electrolysis comprises the steps of:

a. optionally treating the waste material a first time with an ammoniumchloride solution at an elevated temperature to form a first productsolution which comprises dissolved zinc constituents whereby any ironoxide in the waste material will not go into solution;

b. if the first ammonium chloride leach is used, separating the firstproduct solution from the undissolved waste material compounds presentin the first product solution including any of the iron oxide;

c. roasting the undissolved waste material compounds from the firstleach, or roasting the waste material, at an elevated temperature and ina reducing atmosphere to create a roasted waste material compound;

d. treating the roasted waste material compound with the ammoniumchloride solution at an elevated temperature to form a product solutionwhich comprises dissolved zinc constituents whereby any iron oxideremaining in the roasted undissolved waste material compounds will notgo into solution;

e. combining the first product solution, if the first ammonium chlorideleach is used, with the product solution to form a combined productsolution, maintaining the combined product solution at a temperature ofat least 90° C., and adding powdered zinc metal and a dispersant to thecombined product solution whereby any lead and cadmium ions containedwithin the combined product solution are displaced by the zinc metal andprecipitate out of the combined product solution as lead and cadmiummetals and the dispersant is selected from the group consisting ofdispersants which will prevent the aggregation of the zinc metal;

f. separating the combined product solution from the lead and cadmiummetals; and then

g. subjecting the combined product solution to electrolysis to extractzinc metal from said combined product solution.

The combined product solution from the leaching steps comprises zincions in solution as Zn²⁺. When the combined product solution issubjected to electrolysis in an electrolytic cell containing an anodeand a cathode, the zinc metal is electrodeposited on the cathode.Although it is preferable to have the cathode made from zinc metal,cathodes of other material also will allow the electrodeposition of zincmetal from the combined product solution.

Any of the electrolysis cells discussed in the literature are suitable,as long as such cells are configured for the electrolysis of zinc ioncontaining solutions. The two electrodes of the electrolysis cells areconnected externally to a power supply capable of impressing a suitablevoltage across the electrodes. The zinc ions, being positive in nature,migrate toward the negative electrode, or cathode, where they combinewith electrons supplied by the external circuit to form neutral zincmetal atoms. When this happens, the zinc metal, in effect, electroplatesonto the cathode. By using a zinc cathode, the entire cathode can beremoved and used as necessary as a source of zinc. Alternatively, acathode on which electroplated zinc metal can be easily removed can beused.

E. Periodic Precipitation of other Solubles from the Product Solution

The product solution also contains sodium, potassium, magnesium,calcium, and other solubles in solution. These solubles can be recoveredby introducing an electrolyte either in the leaching step or in theammonium chloride storage tanks receiving the recycled product solution.As ammonium chloride is used as the leachant, ammonium salts in solutionis the preferred electrolyte. For example, if some ammonium sulfate isadded, one could precipitate out calcium sulfate. Ammonium sulfate is apreferred electrolyte to add because the process already uses ammoniumin the form of ammonium chloride. The preferred electrolytes includeammonium sulfate, ammonium hydroxide, or ammonium carbonate toprecipitate out various solubles.

F. Recovery of Ammonium Chloride and Wash Water Purification

The wash water used to wash the zinc compounds precipitated from theproduct solution contains some ammonium chloride, as well as othercompounds. Rather than dispose of this polluted wash water, it can betreated to produce pure water and a more concentrated solutioncontaining ammonium chloride and other compounds. The pure water can berecycled to wash additional zinc compounds precipitated from the productsolution, and the concentrated solution can be recycled back to theleaching step. The purification can be accomplished using evaporatorcondensers or reverse osmosis membrane technology.

From an economically competitive situation, the use of reverse osmosismembrane technology to filter the wash water containing ammoniumchloride solution to obtain pure water on one side of the membrane and aconcentrated ammonium chloride solution on the other side of themembrane, will save feed costs. Every so often it will be necessary toback flush the salts off of the membrane to recover them for makeup usein the future. In essence, reverse osmosis membrane technology is usinga pump to pump the wash water through a membrane, which is significantlylower in cost than burning natural gas in an evaporator condenser toevaporate and recondense distilled water. This technology is used tofilter out sodium chloride and the minerals out of sea water to makedistilled water.

II. A Preferred Embodiment of the Zinc Oxide Purification Process

Once the essentially pure zinc oxide has been recovered, as discussedabove, a further zinc oxide purification process is utilized which, in apreferred embodiment, is based on the solubility of zinc oxide in aconcentrated sodium hydroxide solution. As can be seen from FIGS. 3 and4, the solubility of zinc oxide in sodium hydroxide increasessignificantly with increasing sodium hydroxide concentration. Forexample, a 16 molar sodium hydroxide solution (640 g per liter) willdissolve 4 mole (320 g) of zinc oxide. If this solution is then dilutedby a factor of 4, the solubility will decline so that approximately 180g of zinc oxide/zinc hydroxide will precipitate. In accordance with thepreferred embodiment, the zinc oxide purification process utilizes thisphenomenon to produce zinc oxide which is at least 99.8% pure.

In the first step of the preferred process, zinc oxide is dissolved in a50%-70% sodium hydroxide solution. Since most metals are not soluble inconcentrated sodium hydroxide, most of the metal impurities in the zincoxide will not dissolve, including manganese, iron and cadmium. Lead andcalcium are soluble in concentrated sodium hydroxide and therefore willdissolve, as will chloride. The solution is then filtered to remove theundissolved materials which are then sent to the metals recovery sectionof the plant.

The solution is then diluted with water by a factor ranging from 3 to30, but preferably 3 to 8, and optimally around 4, which appears to beoptimum from the point of view of product recovery and energy costs. Thebest mode for the dilution step is performed hot at a temperature at orabove 70° C. and preferably at temperatures ranging from 80 to 100° C.at atmospheric pressure. Temperatures below 70°, and temperatures above100° C. at pressures greater than atmospheric, may be used, but are notas economically as advantageous as in the preferred range. The hottemperatures cause the formation of zinc oxide to be favored over theformation of zinc hydroxide. The resulting zinc oxide crystals whichform are then filtered out, sent to a wash tank where they are washedwith water, and then sent to a dryer where they are dried, preferably ata temperature of 160° C.

The diluted sodium hydroxide solution is then sent to an evaporatorcondenser where the solution is concentrated back to 50%-70% sodiumhydroxide and then reused. When a steady state has been achieved, thisstep results in the formation of sodium chloride crystal which will befiltered out of the solution and recovered. This is because sodiumchloride formed by the chloride present in the zinc oxide is lesssoluble in concentrated sodium hydroxide solution than in dilute sodiumhydroxide. After the sodium chloride is filtered out, the concentratedsolution can be reused in the purification process.

Periodically, lead will be removed from the sodium hydroxide solution bycementation. This involves the addition of zinc dust which will displacethe lead in solution. The lead will then be filtered out and sent to thelead recovery portion of the plant.

By controlling the rate of dilution of the sodium hydroxide solution orits method of addition during the zinc oxide crystallization step, it ispossible to control the particle size hence the surface area of the zincoxide produced. Furthermore, it should be observed that the zinc oxidepurification process is not limited to the purification of zinc oxiderecovered by the zinc oxide recovery process of the present inventionand can be used to purify zinc oxide provided from any source.

Additionally, by selecting the method of addition of the intermediatesolution, preferably sodium hydroxide, during the zinc oxidecrystallization step, it is possible to control the particle size hencethe surface area of the zinc oxide produced. It has been found that thesmaller the droplet size in which the solution is added, the smaller theparticle size (larger surface area). By dispersing the sodium hydroxideinto droplets by a hydraulic atomizer, the particle size can becontrolled. Additionally, at a constant droplet size, vigorous mixingwill result in a larger surface area. The principle can be employedthrough selection of the appropriate droplet size and amount of mixing,to obtain highly purified zinc oxide with a predetermined surface area.This general relationship is shown in Table IV.

TABLE IV Approximate Droplet Size Approximate Surface Area 250 microns 2.0 m²/g 180 microns  3.0 m²/g 150 microns  4.0 m²/g 100 microns 10.0m²/g

FIGS. 3 and 4 illustrate the solubility of zinc oxide in a sodiumhydroxide solution. As shown in FIG. 3, as the concentration of thesodium hydroxide increases, the number of moles of zinc oxide which canbe dissolved in the sodium hydroxide solution increases. As shown inFIG. 4, as the sodium hydroxide solution is diluted, the number of moleswhich can be dissolved in the solution decreases, i.e., the zinc oxidein the solution begins to precipitate.

This solubility characteristic of zinc oxide in sodium hydroxide is usedby the present invention to purify zinc oxide by first dissolving thezinc oxide in a highly concentrated solution of sodium hydroxide andfiltering out the impurities which do not dissolve, and then by dilutingthe sodium hydroxide solution to cause the zinc oxide to precipitate. Bycontrolling the rate of dilution, the particle size and surface area ofthe zinc oxide produced can be controlled.

FIG. 5 illustrates a schematic diagram of the apparatus of the presentinvention for performing the purification process of the presentinvention in accordance with the preferred embodiment to obtain zincoxide which is at least 99.8% pure. A hold tank 10 maintains a 50%-70%NaOH solution at 120° C. to 150° C. The zinc oxide-containing product tobe purified is dissolved in the concentrated NaOH solution in digestiontank 11, which solution is delivered to digestion tank 11 from hold tank10. The undissolved impurities are filtered out at tramp press 13 andthe concentrated solution containing the zinc oxide is delivered toprecipitation tank 14. The solution contained in precipitation tank 14is then diluted at a predetermined rate with distilled water from tank16. As discussed above, preferably the dilution takes place at atemperature ranging from 70° C. to 100° C., and preferably from 80° C.to 100° C., so that the formation of zinc oxide as opposed to zinchydroxide is favored. The zinc oxide crystal precipitates due to thedecreasing solubility of zinc oxide as the NaOH solution is diluted. Thezinc oxide crystal is then filtered and washed with water in zinc oxidepress 18. The zinc oxide crystal is then dried, preferably atapproximately 160° C., in zinc oxide drier 19.

The diluted solution, after the zinc oxide crystal has been filteredout, is collected in feed tank 21 from which it is delivered to anevaporator condenser 22 which concentrates the solution back to 50%-70%.When steady state is achieved, sodium chloride crystals will form whichare filtered out at NaCl press 24 as the re-concentrated NaOH solutionis delivered back to hold tank 10 for reuse in the purification process.

Periodically, lead will be removed from the NaOH solution by cementationby adding zinc dust which displaces the lead in solution. The lead willbe filtered out and sent to the lead recovery portion of the plant. Thepurified zinc oxide can be ground, sized and bagged at station 26.

The following examples illustrate how the purification process inaccordance with the preferred embodiment results in a zinc oxide productwhich is at least 99.8% pure.

EXAMPLE 14

48 grams of NaOH are dissolved in 52 grams of distilled water making a12 molar solution. 21 grams of zinc oxide is added making a saturatedsolution. The excess zinc oxide along with any insoluble impurities isfiltered out. The zinc oxide used was obtained from the recovery processof the present invention and was approximately 1% chloride, 700 ppmmanganese, 150 ppm iron, and 300 ppm lead.

The solution was then added to a volume of boiling water resulting in adilution of 30 times. After boiling for a few minutes zinc oxidecrystals appeared. These crystals were filtered, dried and washed. Theywere then heated for one hour at 160° C. The resulting zinc oxide was afine white powder with a surface area of 7.3 m²/g as measured by the BETmethod. Analysis of the zinc oxide showed no detectable iron ormanganese using DCP analysis. Lead was present at 160 ppm and chloridewas under 50 ppm. The material contained 99.8% or greater zinc oxide.

EXAMPLE 15

The same solution as used in Example 14 was prepared. The solution wasadded to a volume of boiling water resulting in a dilution by a factorof six. Zinc oxide crystals appeared rapidly. These crystals werefiltered, washed, dried and then heated for one hour at 160° C. Theresulting zinc oxide was a fine white powder with no detectable iron ormanganese and a chloride content of less then 50 ppm. The materialcontained 99.8% or greater zinc oxide.

EXAMPLE 16

The solution used in Example 14 was prepared and placed in a one litervessel and kept at 90° C. Water at 90° C. was added slowly over a periodof one hour until the solution was diluted by a factor of five. Theresulting zinc oxide was filtered out and dried for one hour at 160° C.It was a fine white powder with no detectable iron or manganese and achloride content of less then 50 ppm. The material contained 99.8% ormore zinc oxide.

The type of intermediate used in the zinc oxide purification processwill depend on the desired purity and particle characteristics to beobtained. For example, it has been determined that if ammonium sulfateis used as the intermediate instead of sodium hydroxide, the desiredparticle size of the purified zinc oxide can be obtained by controllingthe cooling of the ammonium sulfate solution to precipitate zinchydroxide, because the solubility of zinc oxide in ammonium sulfate istemperature dependent. The following example illustrates how thisembodiment can be used to purify zinc oxide while obtaining desirablesize and shape characteristics.

EXAMPLE 17

A saturated, boiling solution of 100 gm of ammonium sulfate in 100 g ofwater was prepared. Zinc oxide prepared from EAF dust by the ammoniumchloride recovery process discussed above which contains 4% chloride ionwas added to the solution. The saturated solution was filtered,maintaining the temperature above 95° C. The level of chloride in thiszinc oxide affects the quantity dissolved in the ammonium sulfate. Oncooling to 60° C. over 20 minutes zinc hydroxide was precipitated,filtered and washed. After heating to 150° C., 6 gm of zinc oxide havinga surface area of 8 square meters per gram was obtained. The product hasa considerably reduced level of chloride ion (below 0.01%) and also alower lead content.

Intermediates other than sodium hydroxide and ammonium sulfate can alsobe used to precipitate zinc oxide having the desired purity and particlecharacteristics. The following intermediates are chosen in accordancewith the desired purity and particle characteristics to be obtained:ammonium chloride liquor, ammonium phosphate, potassium hydroxide,ammonia/ammonium oxalate and ammonia/ammonium carbonate solutions.

EXAMPLE 18

21 g zinc oxide containing 2% chloride, 200 ppm lead, 100 ppm manganese,50 ppm iron are added to 100 ml of sodium hydroxide solution (50% byweight) at 100° C. The solution was filtered to remove any insolublematerials and then added to 300 ml of water at 100° C. The solution isstirred and the resulting zinc oxide is filtered washed and dried. Thezinc oxide obtained has no measurable lead, manganese or iron presentand has under 50 ppm chloride. The surface area is 1.8 m²/g.

This experiment was then repeated with the sodium hydroxide solutionbeing dispersed into droplets by a hydraulic atomizer. The averagedroplet size of the solution was 180 microns. The resulting zinc oxidehad a surface area of 3.0 m²/g. The composition was the same.

III. Alternative Methods of Producing Iron Feedstocks

Two alternative methods of producing iron feedstocks using thepurification method have been developed. First is a method for theproduction of a feedstock which comprises usable iron constituents and apurified zinc oxide product from industrial waste streams, comprisingthe steps of combining a first waste material stream which is iron poorand comprises non-iron constituents with a second waste material streamwhich comprises iron and non-iron constituents to produce a wastematerial combination, wherein at least one of said waste streamscomprises zinc oxide; roasting the waste material combination at anelevated temperature of at least 500° C. in a reducing atmosphere priorto treating the waste material combination with an ammonium chloridesolution; treating the waste material combination with a sodiumhydroxide solution to form a product solution which comprises dissolvednon-iron constituents and an undissolved precipitate, whereby anynon-leachable metals and metal compounds in the waste materialcombination will be contained in the undissolved precipitate and willnot go into solution; separating the product solution from theundissolved precipitate; roasting the undissolved precipitate at anelevated temperature to reduce any iron oxide in the waste materialcombination into direct reduced iron, resulting in the production of afeedstock which comprises usable iron constituents; diluting the productsolution at a predetermined rate by a factor ranging from 3 to 30 at atemperature ranging from 70° C. to 100° C. to precipitate zinc oxidecrystals; filtering out the zinc oxide crystals and washing the zincoxide crystals in water; and drying the zinc oxide crystals.

Second is a method for the production of a feedstock which comprisesusable iron constituents and a purified zinc oxide product from one ormore industrial waste streams, at least one of which comprises zincoxide, comprising the steps of scrubbing a first waste material streamwhich is iron poor and comprises non-iron constituents; combining thescrubbant and a second waste material stream which comprises iron andnon-iron constituents with a sodium hydroxide solution to form a productsolution which comprises dissolved non-iron constituents and anundissolved precipitate, whereby any non-leachable metals in the wastematerial combination will be contained in the undissolved precipitateand will not go into solution; separating the product solution from theundissolved precipitate; roasting the undissolved precipitate at anelevated temperature to reduce any iron oxide in the waste materialcombination into direct reduced iron, resulting in the production of afeedstock which comprises usable iron constituents; diluting the productsolution at a predetermined rate by a factor ranging from 3 to 30 at atemperature ranging from 70° C. to 100° C. to precipitate zinc oxidecrystals; filtering out the zinc oxide crystals and washing the zincoxide crystals in water; and drying the zinc oxide crystals.

IV. Production of Zinc Compounds from Zinc Oxide

The zinc oxide produced by this process can be used to make a number ofother zinc compounds. Those include, zinc acetate, zinc borate, zincbromate, zinc carbonate, zinc chloride, zinc chromate, zinc hydroxide,zinc nitrate, zinc phosphate, zinc stearate, zinc gluconate, zincsulfate, and zinc EDTA salt. This list is not exhaustive and many otherzinc compounds can be made by adding the appropriate reactants to thezinc oxide slurry.

Commercial zinc oxide is usually made by combustion of zinc vapor in airand is collected as a dry powder. The zinc oxide prepared as describedherein is a precipitate in aqueous solution. This allows a range ofdownstream chemicals to be manufactured by addition of the appropriateacid to the zinc oxide slurry. Well known methods of producing zinccompounds by the addition of the appropriate acid can be used in thecurrent invention.

The zinc compounds can be manufactured by the invention directly withouthaving to suspend or dissolve the dry zinc oxide. Synthesis of zinccompounds by this method also obviates the need to dry zinc oxideobtained from the purification process described above. Both simplecommodity chemicals and specialty products having a particular physicalor chemical properties can easily be made by employing this process inconjunction with the methods to control particle size of the zinc oxidedescribed above.

EXAMPLE 19

A stoichiometric quantity of sulfuric acid was added to the zinc oxideaqueous slurry. The zinc oxide dissolved to give a zinc sulfatesolution. Zinc sulfate solution is used in rayon manufacture and inagriculture.

EXAMPLE 20

A slight stoichiometric excess of stearic acid was added to the zincoxide aqueous slurry. It was heated with agitation to 60° C. to melt thestearic acid. As the melt temperature is approached, the stearic acidreacted with the zinc oxide to produce fine particle size zinc stearate.Fine particle size zinc stearate is used in the rubber and paintindustries.

The above description sets forth the best mode of the invention as knownto the inventor at this time, and the above Examples are forillustrative purposes only, as it is obvious to one skilled in the artto make modifications to this process without departing from the spiritand scope of the invention and its equivalents as set forth in theappended claims.

What is claimed is:
 1. A continuous method for the recovery of zincoxide from waste material streams which comprise zinc compounds,comprising the steps of: a. roasting said waste material at an elevatedtemperature and in a reducing atmosphere; b. treating said wastematerial with an ammonium chloride solution at an elevated temperatureto form a product solution which comprises dissolved zinc and dissolvedzinc oxide whereby any iron oxide in said waste material will not gointo solution; c. separating said product solution from any undissolvedmaterials present in said product solution including any of said ironoxide; d. adding zinc metal and a dispersant to said product solutionwhereby any lead and cadmium ions contained within said product solutionare displaced by said zinc metal and precipitate out of said productsolution as lead and cadmium metals and said dispersant is selected fromthe group consisting of dispersants which will prevent the aggregationof said zinc metal; e. separating said product solution from the leadand cadmium metals; f. lowering the temperature of said product solutionthereby precipitating a mixture of crystallized zinc compounds; g.separating said precipitated zinc compounds from said product solution;h. washing said precipitated zinc compounds with wash water therebysolubilizing certain of said precipitated zinc compounds; i. separatingremaining precipitated zinc compounds that have not solubilized fromsaid product solution; j. drying said remaining precipitated zinccompounds at a temperature of at least 100° C. whereby a resultingproduct is zinc oxide of 99% or greater purity by weight; k. dissolvingsaid resulting product in a concentrated sodium hydroxide solution; l.filtering out any undissolved materials; m. dispersing said sodiumhydroxide solution into droplets between 100 and 300 microns in size; n.combining said droplets with a sufficient amount of 70° C. to 100° C.water to dilute the sodium hydroxide solution by a factor of 3 to 30 byvolume, thereby precipitating zinc oxide crystals; and o. filtering outsaid zinc oxide crystals.
 2. The method of claim 1 further comprisingthe steps of: p. washing said zinc oxide crystals in water; and q.adding an acid to said zinc oxide crystals; wherein said acid reactswith the zinc oxide to form a zinc compound.
 3. The method of claim 2,wherein said acid is selected from the group consisting of acetic acid,boric acid, bromic acid, carbonic acid, chromic acid, nitric acid,phosphoric acid, stearic acid, gluconic acid, hydrochloric acid,sulfuric acid, and edetic acid.
 4. The method of claim 1, wherein, instep (h), said wash water is above 25° C.
 5. The method of claim 4,wherein said wash water is at a temperature of 60° C. to 100° C.
 6. Themethod of claim 5, wherein said zinc compounds are washed with water ata ratio of between 0.1 and 2 pounds of zinc compounds per gallon of washwater.
 7. The method of claim 6, wherein the temperature and the ratioof water to zinc compounds is controlled to obtain zinc oxide crystalshaving a desired surface area.
 8. The method of claim 1, wherein, instep (m), said droplets are between 150 and 250 microns in size.
 9. Themethod of claim 1, wherein the size of said droplets in step (m) iscontrolled to obtain zinc oxide crystals having a desired surface area.10. The method of claim 1, wherein, in step (n), said amount of water issufficient to dilute the solution by a factor of 3 to 8 by volume. 11.The method of claim 1, wherein, in step (n), said water is at atemperature ranging from 90° C. to 100° C.